A number of bacterial small RNAs (sRNAs) act as global regulators of stress
responses by controlling expression of multiple genes. The sRNA SgrS is expressed
in response to glucose-phosphate stress, a condition associated with disruption of
glycolytic flux and accumulation of sugar-phosphates. SgrS has been shown to
stimulate degradation of the ptsG mRNA, encoding the major glucose transporter.
This study demonstrates that SgrS regulates the genes encoding the mannose and
secondary glucose transporter, manXYZ. Analysis of manXYZ mRNA stability and
translation in the presence and absence of SgrS indicate that manXYZ is regulated by
SgrS under stress conditions and when SgrS is ectopically expressed. In vitro
footprinting and in vivo mutational analyses showed that SgrS base pairs with
manXYZ within the manX coding sequence to prevent manX translation. Regulation
of manX did not require the RNase E degradosome complex, suggesting that the
primary mechanism of regulation is translational. An Escherichia coli ptsG mutant
strain that is manXYZ(+) experiences stress when exposed to the glucose analogs α-
methyl glucoside or 2-deoxyglucose. A ptsG manXYZ double mutant is resistant to
the stress, indicating that PTS transporters encoded by both SgrS targets are
involved in taking up substrates that cause stress. We further demonstrate that SgrS
binds to two sites on the manXYZ mRNA to coordinately down-regulate translation
of all the cistrons, a mechanism reminiscent of that used by the eukaryotic miRNA,
lin-4 for regulation of the lin-14 mRNA. We tested the hypothesis that regulation of
manY and manZ is dependent on SgrS:manX base pairing and subsequent manXYZ
mRNA degradation. Contrary to the hypothesis, we show that SgrS repression of
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manY and manZ translation can be decoupled from SgrS-dependent manXYZ mRNA
degradation. Furthermore, SgrS regulates manY translation by a mechanism that is
independent of SgrS:manX mRNA interactions. Instead, translational regulation of
manY depends on SgrS pairing with sequences in the manX-manY intergenic region,
upstream of the manY ribosome binding site. Mutational analysis suggests that
while the SgrS-manY interaction is sufficient to stimulate some degradation of the
manXYZ mRNA, pairing with both sites is required for maximal degradation of the
manXYZ mRNA. Additional SgrS candidate-mRNA targets are also investigated.